1. Field of the Invention
The present invention concerns a method and line for repairing a closure fault of a metal tube containing at least one transmission optical fiber.
2. Description of the Prior Art
Transmission optical fibers, especially in a submarine optical fiber cable, are contained in one or more metal (for example steel) tubes providing sealed mechanical protection around the fibers within the structure of the cable.
The operation of inserting an optical fiber into a metal tube is known in itself and is described in document EP-A-0 299 123, for example. It is carried out continuously on a fiber insertion line. It consists in deforming a flat metal strip into the shape of a longitudinally open tube, guiding the fiber into the open tube and closing the tube lengthwise by laser welding the edges of the opening. An auxiliary tube enters the open metal tube and extends within the latter beyond the laser. It introduces and guides the fiber into the metal tube.
This operation is usually followed by a check on correct closure of the metal tube to achieve the required protection of the fiber. This detects any closure defects along the tube and memorizes their position, so that the defects can be repaired later.
A closure defect in the tube can be repaired by fitting an external sleeve to the defective area. This leads to a localized increase in the overall size of the repaired tube, which is often incompatible with its use, especially Within the structure of a submarine cable.
A repair can instead be effected by cutting the metal tube and the fiber on either side of the defective area, which is removed, and then making a joint.
Document U.S. Pat. No. 5,076,657 describes a joint of this kind, which does not cause any increase in the overall size of the tube at the location of the joint. The joint is made using a sleeve to protect the ends of the cut fiber, which are welded together, and to re-establish the continuity of the cut tube. The sleeve is welded to the end parts to be joined of this tube and its diameter is made the same as the outside diameter of the tube by mechanical means. The joint can instead be made with no sleeve, by reducing the outside diameter of one of the end parts of the cut tube, inserting this end part into the other end part of the cut tube and welding them together. In a joint of this kind the inside dimensions of the repaired tube are reduced where the two cut end parts overlap.
These prior art repair methods are time-consuming and difficult. They entail the risk of damage to the interior fiber. Also, the same metal tube often contains a plurality of transmission optical fibers, which makes repairing the metal tube closure defect even longer and more difficult
An object of the present invention is to avoid the above-mentioned disadvantages, to carry out the repair quickly, easily and reliably, without modifying the outside and/or inside dimensions of the tube, and with the minimum risk of deterioration of the transmission characteristics of the fiber or fibers inside the tube.